It is known that fuel cell stacks produce electrical energy directly via an electrochemical redox reaction using hydrogen (the fuel) and oxygen (the oxidant) without passing via a mechanical energy conversion step. This technology seems promising, especially for motor vehicle applications. A fuel cell stack comprises in general the series combination of unitary elements each consisting essentially of an anode and a cathode separated by a polymeric membrane allowing ions to pass from the anode to the cathode.
It is very important to have permanently available a precise evaluation of the permeability of the ion-exchange membrane of each cell of a fuel cell stack so as to monitor its state of ageing and to be able to stop the use of the cell if safety were to be compromised. Although the principle of measuring the permeability of an ion-exchange membrane by pressure difference is conventional, in practice only investigatory methods are known that dictate the use of appropriate equipment and a manual procedure. For example, an external nitrogen bottle is used which is connected to one of the circuits, either the anode or the cathode circuit, and the loss of gas to the other circuit is observed.
Patent application US 2004/124843 provides a method for determining the individual permeability of each ion-exchange membrane of a fuel cell stack. To do so, the anode is supplied with hydrogen and the cathode is supplied with nitrogen or with another inert gas. According to the Nernst's equation, the difference in nature of the gas on either side of the membrane generates a potential difference that depends inter alia on the nature and the concentration or partial pressure of these gases. It appears that if a membrane is particularly permeable, the hydrogen will diffuse on the cathode side, and vice versa, thus modifying the nature of the gas mixture on either side of the membrane and consequently also modifying the potential difference measured on this cell. The method entails a voltage measurement, a measurement of the pressure within the anode circuit and of the pressure within the cathode circuit and a temperature measurement in order to solve the Nernst's equation, so as to detect whether one or more membranes mounted within a fuel cell stack have a permeability defect.
However, this method suffers from the following implementation difficulties:                the theoretical potential difference with pure hydrogen at the anode and pure nitrogen at the cathode is at most a few tens of mV, which implies a very accurate voltage measurement apparatus;        determination of the permeability involves flow rate measurements, which in practice are difficult to carry out with great precision for gas mixtures;        the slightest trace of residual oxygen at the cathode may generate a voltage difference much higher than the expected voltage level and therefore falsify the measurement, yet it is well known that in practice it is very difficult to guarantee complete disappearance of a gas, most particularly in the presence of an absorbent support such as the GDL (gas diffusion layer) contained in a membrane electrode assembly (MEA); and        finally, this method involves a particular way of conditioning the system and requires a source of nitrogen or another inert gas being available. The method is therefore difficult to automate, most particularly in the context of onboard applications.        
Patent application WO 2006/012954 discloses a procedure with no nitrogen injection phase, including a phase of venting the cathode circuit to atmosphere. The fuel cell stack described in that text does not contain a booster pump for air injection. It follows that the pressure at the cathode cannot be raised above atmospheric pressure. The pressure differential with the anode will therefore be insufficient for carrying out the automatic membrane permeability measurement. In addition, evaluating the permeability of the ion-exchange membranes of a fuel cell stack remains completely outside the disclosure of this document.
Patent application US 2009/0220832 proposes a fuel cell stack comprising a recirculation loop to the cathode and to the anode, and valves for isolating the internal circuits of the stack from the atmospheric air. However, the proposed arrangement of the components and the procedure described are intended to inundate the circuits of the stack with practically pure hydrogen, which is neither safe nor economic. Moreover, evaluation of the permeability of the ion-exchange membranes of a fuel cell stack remains entirely outside of the disclosure of this document.